Comfort rating to give yourself a safety margin and insure the insulation under your sleeping bag has a R rating of at least 4.5 when using a synthetic sleeping bag.
Unless you are tough like the russian climbers who just flake out their rope for use as a mat.
It also depends quite a lot on what clothing you wear inside the bag.
Ed Viesturs describes in one of books using a very light bag on Mt Everest. He wears his full body down climbing suit inside a light bag. He does anything he can to save a few grams of weight.
For those who don't follow climbing, Viesturs is the first American to climb all 14 of the 8000 meter peaks, and the fifth person to do so without using bottled oxygen. He has climbe Mt Everest seven times. He is one very hard dude who has spent a lot of time in extreme cold. Don't try this at home, kids.
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